The thermally excited triplet state ofp-phenylenebis(phenylmethylene) (p-PBPM) which is a topological isomer of the first organic high-spin molecule,m-phenylenebis(phenylmethylene) (m-PBPM, S=2), was examined using singlecrystal1H-ENDOR (Electron-NuclearDOubleResonance) spectroscopy. From the1H-ENDOR spectrum of the triplet state we found that the absolute sign of the fine-structure parameterDwas negative and thatp-PBPM in the triplet state has eight negative π-carbon sites and six positive ones. The π-spin density distribution determined experimentally indicates that the carbon atoms of the centralp-phenylene ring have negative π-spin density. The spin-spin interaction between the induced π-spins causes the fine-structure parameter D to be negative. It is concluded that the π-π spin interaction predominates in the triplet state ofp-PBPM.